A technique has been known in which an image encompassing a wide subject region beyond an imaging area captured by one camera is generated by combining multiple captured images together. For example, Patent Literature 1 discloses a technique that performs a correction for allowing a concentration characteristic of one image B to approximate a concentration characteristic of the other image A on the basis of the respective concentration characteristics of the images A and B in an overlapping portion of the images A and B adjacent to each other.
Specifically, according to the disclosure of Patent Literature 1, an overlapping portion of the image A with the image B is cut out as a composite part a, and an overlapping portion of the image B with the image A is cut out as a composite part b. Then, histograms Ha(x) and Hb(x) of the respective composite parts a and b are generated from pixel values of the composite parts a and b. Then, gradation conversion tables LUTa and LUTb that can execute histogram equalization (histogram equalization) are generated for the respective histograms Ha(x) and Hb(x). Then, gradation conversion is executed on an overall area of the image B with the use of the gradation conversion table LUTb, and reverse gradation conversion is executed on the image subjected to the gradation conversion with the use of the gradation conversion table LUTa. In other words, a gradation conversion process for allowing the histogram Hb(x) of the composite part b to approximate the histogram Ha(x) of the composite part a is subjected to the overall area of the image B.
The technique disclosed in Patent Literature 1 described above is applied to a composite image in which multiple images are continuously arranged in a strip (in a predetermined direction). Therefore, if one of the multiple captured images is used as a reference, and the other captured images are corrected in turn, deviations in the concentration characteristic between the respective adjacent images can be suppressed in all of the boundary portions (portions where the respective images are adjacent to each other) of the composite image.
On the other hand, as the composite image, there is also a composite image in which multiple images are continuously arranged in an annular shape, for example, like an image (so-called top view image) of surroundings of the vehicle viewed from a top. In this case, the technique disclosed in Patent Literature 1 suffers from such a problem that the deviations in the concentration characteristic between the respective adjacent images cannot be suppressed in one or more boundary portions of the composite image.
In other words, for example, when four images A, B, C, and D are continuously arranged in a stripe, the image B may be corrected on the basis of the image A, the image C may be corrected on the basis of the corrected image B, and the image D may be corrected on the basis of the corrected image C.
On the contrary, when the four images A, B, C, and D are continuously arranged in an annular shape, the deviation in the concentration characteristic occurs in a boundary portion where the image A is adjacent to the corrected image D by merely correcting the images B, C, and D in order as with a case of the stripe. If the image A is corrected on the basis of the corrected image D, the deviation in the concentration characteristic between the image A and the image D is suppressed. However, because the deviation in the concentration characteristic between the corrected image A and the image B occurs instead, the above problem is not eliminated. The above problem is not limited to the image of the surroundings of the vehicle viewed from the top, but may arise in a composite image in which the multiple images are continuously arranged in the annular shape.